Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS), referred to as Node Bs with regard to universal mobile telecommunication system (UMTS) systems, and a plurality of subscriber units, often referred to as user equipment (UE) in UMTS systems.
The communication link from a Node B to a UE is generally referred to as a downlink communication channel. Conversely, the communication link from a UE to a Node B is generally referred to as an up-link communication channel.
In an UTRA-based wireless communication system, each Node B has associated with it a particular geographical coverage area (or cell). The coverage area is defined by a particular range over which the Node B can maintain acceptable communications with UEs operating within its serving cell. Often these cells combine to produce an extensive coverage area.
In such wireless communication systems, methods exist for communicating information simultaneously where communication resources in a communication network are shared by a number of users. Such methods are termed multiple access techniques. A number of multiple access techniques exist, whereby a finite communication resource is divided into any number of physical parameters, such as:
(i) Frequency division multiple access (FDMA) whereby frequencies used in the communication system are shared,
(ii) Time division multiple access (TDMA) whereby each frequency used in the communication system, is shared amongst users by dividing the communication resource (each frequency) into a number of distinct time periods (time-slots, frames, etc.), and
(iii) Code division multiple access (CDMA) whereby communication is performed by using all of the respective frequencies, in all of the time periods, and the resource is shared by allocating each communication a particular code, to differentiate desired signals from undesired signals.
Within such multiple access techniques, different duplex (substantially simultaneous two-way communication) paths are arranged. Such paths can be arranged in a frequency division duplex (FDD) configuration, whereby a first frequency is dedicated for up-link communication and a second frequency is dedicated for down-link communication.
Alternatively, the paths can be arranged in a time division duplex (TDD) configuration, whereby a first time period is dedicated for up-link communication and a second time period is dedicated for down-link communication within the same frequency channel. In addition, some communication channels are used for carrying traffic and other channels are used for transferring control information, such as call paging, between the base station and the subscriber units.
Wireless communication systems are distinguished over fixed communication systems, such as the public switched telephone network (PSTN), principally in that mobile stations/subscriber equipment move between coverage areas served by different Node B (and/or different service providers). In doing so, the mobile stations/subscriber equipment encounter varying radio propagation environments. In particular, in a mobile context, a received signal level can vary rapidly due to multipath and fading effects.
The present invention will be described with respect to the 3rd Generation Partnership Project (3GPP) technical specification ‘TS25.224’ for a wireless communication system based on the universal mobile telecommunications standard (UMTS). UMTS is a CDMA-based system. A CDMA system employs spread spectrum signaling. Two categories of spread spectrum communications are direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS).
In the case of a DSSS communication system, for example, the spectrum of a signal can be most easily spread by multiplying it with a wide-band pseudo-random code generated signal. It is essential that the spreading signal be precisely known so that the receiver can de-spread the signal. A cellular communication system using DSSS is commonly known as a Direct Sequence Code Division Multiple Access (DS-CDMA) system, one example of which is defined in the TIA-EAI standard IS95.
Individual users in the system use the same radio frequencies and time slots, but they are distinguishable from each other by the use of individual spreading codes. Hence, multiple communications channels are allocated using a number of spreading codes within a portion of the radio spectrum. Each unique code is assigned to a UE, except for common channels.
One feature associated with most wireless communication systems, which is particularly needed in a UTRA system, allows the transceivers in the Node B and UE to adjust their transmitter output power to take into account the geographical distance between them. The closer the UE is to the Node B's transceiver, the less power the UE and the Node B's transceivers are required to transmit, for the transmitted signal to be adequately received by the other communication unit. This ‘power control’ feature saves battery power in the UE and also helps to reduce the level of potential interference within the communication system. Initial power settings for the UE, along with other control information, are set by the information provided on a beacon physical channel for a particular cell.
The 3GPP specification assumes a downlink shared channel (DSCH) call model that allows for the implementation of slow measurement-report-based power control of Physical Downlink (DL) Shared Channels (PDSCH's). In such a scheme, the user equipment (UE) is requested to send sporadically measurement reports from which the current mean pathloss between node-B and UE may be determined. In addition, the UE may send interference power measurements. This DL power control scheme is termed “slow” due to the delay in the UE making the measurement and conveying this to the RNC entity via the node-B and due to the measurement reports being sent every few seconds. Measurement reports are sent in gaps between radio link control (RLC) messages that are typically used by a UE to request re-transmission of information (data packets) received in error. This is in contrast with fast (frame or sub-frame) based power control typically applied to dedicated downlink physical channels (DPCHs).
It is known that accurate power control is a vital element of CDMA systems as the spreading codes are not orthogonal on the reverse link. Hence, any error in the power control (PC) levels introduces interference that directly reduces system capacity.
Furthermore, it is known that the 3GPP standard is particularly sensitive to power control mismatches in the up-link because of fast fading effects in the communication channel. Fast fading is a known and generally undesirable phenomenon caused by the signal arriving at a receiver via a number of different paths. Therefore, in order to achieve maximum up-link capacity in a CDMA system, fast power control loops are required.
An inner power control (PC) loop is provided to adjust a UE's transmission power to counter the so-called “near-far” problem. The inner power control loop adjusts the transmission power of each connection such that the received signal power observed at the Node B is sufficient to meet a particular quality of service (QoS) requirement of each particular connection; thereby reducing interference to others in the system. The inner PC loop adjusts the UE's transmission power in order to keep the received reverse link signal-to-interference ratio (SIR) as close to constant as possible.
The predetermined threshold, to which the inner loop SIR measure is compared, is generated by the outer, quality-driven, power control loop. This loop sets a target SIR threshold that is proportional to the required quality of service (QoS) for a given connection (usually defined in terms of target bit error rate (BER) or frame erasure rate (FER)). This target will vary as propagation conditions change, for example as a function of a UE's speed and its specific propagation environment, as both have a major impact on the SIR required at the Node B to maintain the desired QoS.
A reduction in interference is therefore desirable and, from a system-wide perspective, power control can therefore be used in order to maximise the system capacity. If the allocation of power amongst users is carefully managed so as to provide only ‘just enough’ signal quality at the receiving end then intercell interference power will be minimised since too much quality effectively equates to too much power and hence reduced capacity.
Power control can also be employed from a pure link-level-performance perspective in order to mitigate the detection impairments caused through temporal variations in received signal power as a result of the mobile radio propagation channel. If these variations can be removed via effective power control then the required mean SIR at the receiver, necessary to attain a certain bit or block error rate, can be shown to be less than would be required in a fading channel without power control. Thus, if every user can then operate at a lower SIR, system interference is reduced and system capacity is again increased.
Effective power control therefore constitutes an important aspect of overall system design for high-capacity spectrally efficient CDMA deployments.
The power required of a transmitter in order to attain a certain radio link quality (in terms of bit error- or block error rate) is a function of four primary variables:
(i) The pathloss between transmitter and receiver;
(ii) The degree and performance of the error correction (channel coding) scheme employed;
(iii) The prevailing channel propagation conditions (e.g. speed, multipath); and
(iv) The data rate transmitted.
Power control is normally employed to track changes in (i)-pathloss and (iii)-channel propagation conditions, since these processes are not under the control of the system operator. However, the degree of error protection and the data rate transmitted are under control of the system operator and this will affect the required amount of transmitted power.
The preferred embodiment of the present invention is described with regard to implementation on the UMTS Radio Access Network (UTRAN) protocol architecture 100, an overview of the pertinent portions of which is described with regard to FIG. 1. The focus of the preferred embodiment of the invention relates to communication between the medium access layer (MAC) (Layer-2) 110 and the physical layer (PHY) (Layer-1) over transport channels, which are the channels over which data is communicated between the MAC 110 and the PHY. The UMTS Radio Access Network (UTRAN) protocol architecture at layer-2 utilises the concept of Transport Channels 140, 142, 144 to control the bit rate and the forward error correction (FEC) scheme that is employed.
Transport Channels 140, 142, 144 may contain one or more Transport Formats 150, 152, 154, 156, 158, 160 that are characterised by two parameter sets:
(i) A semi-static part that is associated with the Transport Channel to which it belongs. This parameter set defines the type of channel coding to be used, the Transmission Time Interval (TTI), the Static Rate Matching Attribute, and the cyclic redundancy code (CRC) length.
(ii) A dynamic part that is specific to the Transport Format. This parameter set defines the Transport Block size and the Transport Block Set Size, which is equal to the Transport Block Size multiplied by the number of Transport Blocks to be transmitted within the TTI.
Thus, all Transport Formats 150, 152, 154, 156, 158, 160 within the same Transport Channel 140, 142, 144 inherit the same semi-static part, though each of those formats 150, 152, 154, 156, 158, 160 may have a different dynamic part. Transport Formats are identified by labels termed Transport Format Indicators (TFIs).
Coded Composite Transport Channels (CCTrCHs) may be formed by multiplexing one or more Transport Channel processing chains within a multiplexer 170 within Layer-1. The multiplexed output is mapped to an amount of physical resource 180, and in this manner, multiple Transport Channels may be multiplexed onto the same physical resource. This combination of Transport Formats (TFIs) is termed a Transport Format Combination (TFC).
The set of valid TFCS (as configured by layer-3) is termed a Transport Format Combination Set (TFCS) and is notified to the MAC 120. Furthermore, the set of allowed TFCS within the TFCS might be restricted based on factors such as:
(i) The Puncturing Limit (PL), as set by Layer 3;
(ii) The amount of physical resource allocated 125; and
(iii) The amount of transmission power required for the TFC.
Higher layers, or lower layers, than the MAC layer 110, may impose these restrictions 130. Either way, the MAC 110 is informed of the TFCS restrictions 130. The MAC 110 in both the radio network controller (RNC) and the user equipment (UE) is then wholly responsible for selection of a TFC from within the resulting allowed set. The selection of a TFC from within this allowed set is generally based on optimisation of the data volume to be transmitted within the constraints of the physical resource allocated. Selection or changing of the current TFCS is managed by higher layers (L3).
Typically, all TFCs within a TFCS require nominally the same signal quality to attain a given bit or block error rate. Layer-3 makes decisions on the TFCS to use, based on information gathered from measurement reports or other metrics. Adjustment of the transmission rate, per physical resource unit, is therefore primarily governed by Layer-3 decisions via appropriate selection of TFCS. This is shown by TFC selection control function 135 within the MAC layer, with TFC selection control input 138 to the Transport channel formatting within Layer-1.
As the amount of error protection in transmissions is reduced, so the available information rate is increased, since fewer parity (or redundancy) bits must be transmitted. However, as the error protection scheme is weakened, so the received energy per bit (Eb), relative to the receiver noise spectral density (N0) required to achieve a certain error rate, will increase. Hence, the required transmit power will also increase, which is known as a reduction in the coding gain.
The received signal power (S) is:S=Eb*R  [1]
where R is the information rate in bits/sec.
The noise power (N) is:N=N0*W  [2]
where W is the receiver bandwidth in Hz.
Hence, the received signal to noise power ratio is simply:
                              S          N                =                                            E              b                                      N              0                                ×                      R            W                                              [        3        ]            
It can be seen from equation [3] that the required received signal to noise ratio to achieve a certain Eb/N0 increases linearly with the bit rate R (given a fixed system bandwidth W). However, the Eb/N0 required to achieve a certain block error rate is a function of the type and amount of coding used and of the prevailing propagation channel conditions.
Thus, as less error protection is applied to a signal, the required transmit power increases for two reasons:
(i) The coding gain is less (higher Eb/N0 is required for a given error rate); and
(ii) The information rate (R) is increased, from say, 100 kbits/sec to 200 kbits/sec.
Appropriate selection of the transmission rate (TFCS) is therefore tightly coupled with the power control scheme, since both direct power control and selection of TFCS will affect the transmitted power, and thereby the system capacity. Since power is the shared resource in CDMA systems, TFCS must be tightly managed, in conjunction with power control, in order to maximise system capacity.
When the node-B transmits at full power, many UEs in favourable cell locations will see large values of carrier signal to noise plus interference (C/(N+I)), resulting in excessive (too good) quality for those UEs. Excess quality is undesirable from a network capacity perspective since it implies that unnecessary interference is being injected into the system, or conversely that a sub-optimal data rate is obtained for the transmit power being used.
Two mechanisms can be used to reduce this excess quality:
(i) Reduce the power transmitted to UEs operating with excess C/I (i.e. use downlink power control). In this case, the data rate per code to the UE will stay the same. The quality target of the link is still maintained although the amount of interference to other cells is reduced.
(ii) Decrease the processing gain available to those UEs by increasing the bit rate per code. This is achieved by reducing the amount of forward error correction (FEC) protection applied to the data, before transmission. A number of TFCSs may be employed for this purpose, each having varying degrees of FEC protection. In this case, the data rate to the UE is increased, the quality target is still attained, but the amount of interference generated is not reduced since the power of the transmission has not been reduced.
This link between power control and transport format selection is illustrated in FIG. 2, where a selection of transport format is made, from a number of variable transport formats 230, 240, 250 based on the available carrier to interference (C/I) 210. The C/I required for the low rate 230, medium rate 240 and high rate 250 leaves corresponding various attenuation levels 235, 245 that can be imparted onto the transport formats using power control. As shown, as an example, for a high rate transport format 250, there is no room for attenuation by power control 220 to achieve a reduction in C/I 210.
Hence, in order to provide a transport format that would serve a to-be-transmitted packet data transmission 215 as shown, the medium rate transport format 240 would be selected as this delivers the highest data rate for the C/I available 245. The highest rate transport format 250 is unavailable whereas the lowest rate transport format provides sub-optimal data rate 230 for the available C/I 235. The inventor of the present invention has recognised that slow measurement report-based power control is less than adequate for shared channels.
In general, for shared packet data-based systems it is therefore preferable to maintain full, or close to full transmit power from the node-B whilst maximising the data rate to each user since for datavolume driven applications such as web-browsing and file-transfer, every user benefits from every other user receiving the best data rate possible, at any particular moment in time. This is because physical channel resource is liberated and may be used by other users.
In summary, the 3GPP specifications assume a downlink shared channel (DSCH) call model that allows for the implementation of slow measurement-report-based power control of Physical Downlink (DL) Shared Channels (PDSCHs). In such a scheme, the user equipment (UE) is requested to send measurement reports from which the current mean pathloss between node-B and UE may be determined. In addition, the UE may send interference power measurements. This DL power control scheme is termed “slow” due to the delay in the UE making the measurement and conveying this to the RNC entity via the node-B.
As the PC scheme is relatively slow, it provides a less than optimal solution in PDSCHs. This results in increased interference and sub-optimal use of the available communication resource.
A need therefore exists, in general, for an improved power control arrangement and method of operation, and in particular, an arrangement and method for improved downlink power control for shared channels in an UTRA-TDD system, wherein the above-mentioned disadvantages may be alleviated.